Method for preparing positive cathode material for lithium cell, and lithium cell

ABSTRACT

A method for producing a cathode material for a lithium battery, characterized in that it comprises admixing a compound liberating a phosphate ion in a solution and metallic iron, and dissolving the metallic iron, followed by firing, thereby synthesizing ferric phosphate. The above method further comprising reacting a raw material mixture while grinding it down or refluxing can produce ferric phosphate cathode material having a fine particle diameter and exhibiting high activity, through a precursor before firing having a fine particle diameter.

TECHNICAL FIELD

The present invention relates to a method for producing a cathodematerial for a lithium battery and to a lithium battery (primary orsecondary battery) using the cathode material as a constituentcomponent. More specifically, the present invention relates to a methodfor producing a cathode material (FePO₄) for primary and secondarybatteries such as metal lithium battery, lithium ion battery and lithiumpolymer battery using an alkali metal such as lithium or an alloy orcompound thereof as an anode active material and to a lithium primary orsecondary battery using the cathode material produced by the method.

BACKGROUND ART

Primary and secondary batteries such as lithium battery, lithium ionbattery and lithium polymer battery using an alkali metal such aslithium or an alloy or compound thereof as an anode active material areattracting attention in recent years because of their large capacities.The cathode material for use in such primary and secondary batteries issubjected to electrode oxidation/reduction accompanied bydoping/undoping of ions of an alkali metal such as lithium during theprocess of charging and discharging. As the cathode material, the ferricphosphate (FePO₄) having a trigonal P₃₂₁ crystalline structure iswell-known in the prior art (Japanese Patent No. 3126007). AlthoughJapanese Patent No. 3126007 discloses a method for obtaining ferricphosphate anhydride by heat treatment of ferric phosphate hydrate (FePO₄nH₂O), there in no description of the method for synthesizing ferricphosphate hydrate. An example in which a FePO₄ cathode active materialhaving a trigonal P₃₂₁ structure was synthesized from NH₄H₂PO₄ andFe(NH₄)₂(SO₄)₂ 6H₂O at a temperature of 650° C. has been reported [P. P.Prosini et al, J. Electrochem. Soc., 140, A297 (2002)] has beenreported. However, the material has as low a capacity as 40 mAh/g.

Conventionally, ferric phosphate hydrate as a calcination precursor issynthesized by, for example, mixing a solution containing a tervalentiron such as a solution of iron (III) sulfate or ferric chloride (or ahydrate thereof) with an alkaline compound containing phosphate ionssuch as disodium hydrogenphosphate, allowing the reaction mixture tostand under an elevated temperature, and filtering the precipitate. Theferric phosphate hydrate, however, is not suitable for a material ofcathode for a secondary battery because nonvolatile elements such assodium ions tend to remain as impurities. That is, in the synthesismethod, it is necessary to remove sodium ions and so on from thecalcination precursor by filtering. The process is cumbersome and maybring the entry of impurities. To complete the filtering and increasethe purity of the calcination precursor, it is preferred to allow thecrystals of ferric phosphate hydrate precipitate to grow until theyreach a large diameter (about 10 μm or greater). However, when ferricphosphate hydrate particles having a large diameter are calcined, theresulting ferric phosphate particles have a large diameter and have lowactivity as a cathode material. It is known that the performance of acathode material is largely affected by the size, shape and specificsurface area of the particles thereof and impurities therein.

It is, therefore, an object of the present invention to provide a methodfor producing a cathode material by which ferric phosphate suitable as acathode material for lithium batteries such as lithium primary andsecondary batteries can be synthesized reliably and easily, and toprovide a high-performance lithium battery primary or secondary battery)using the cathode material obtained by the method.

DISCLOSURE OF THE INVENTION

In order to solve the above problem, the first aspect of the presentinvention is a method for producing a cathode material for a lithiumbattery, including the steps of mixing a compound which releasesphosphate ions in a solution with metal iron to cause dissolution andreaction of the metal iron, and calcining the reaction mixture tosynthesize ferric phosphate.

According to the method for producing a cathode material for a lithiumbattery, a cathode material (that is, ferric phosphate as a cathodeactive material) can be synthesized from stoichiometric amounts ofingredients reliably and easily. Also, the reaction of “a compound whichreleases phosphate ions in a solution” with metal iron can be carriedout in an aqueous solution and is thus easy to handle. Further, sincethe iron of the cathode material, unlike that of an olivine-type (Pnmacrystal structure) lithium iron (II) phosphate known as another cathodematerial, is oxidized to Fe+3 by calcination, the calcination can becarried out in the presence of air. Thus, the calcination processrequires no special conditions such as a reducing atmosphere of hydrogenand is thus easy to carry out.

The second aspect of the present invention is a method for producing acathode material for a lithium battery, including the steps of reactinga compound which releases phosphate ions in a solution with metal ironwhile grinding the mixture in an aqueous solution, and calcining thereaction mixture to synthesize ferric phosphate.

According to the method for producing a cathode material for a lithiumbattery, ferric phosphate as a cathode material (cathode activematerial) can be synthesized from ingredients at a stoichiometric ratioreliably and easily. Also, the reaction of “a compound which releasesphosphate ions in a solution” with metal iron can be carried out in anaqueous solution and is thus easy to handle. In addition, since theingredient mixture is ground during the reaction, the reaction can beaccelerated

Further, since iron of the cathode material of the present invention,unlike that of an olivine-type lithium iron(II) phosphate known asanother cathode material, is oxidized to Fe+3 by calcination, thecalcination can be carried out in the presence of air. Thus, thecalcination process requires no special condition such as a reducingatmosphere of hydrogen and is thus easy to carry out.

The third aspect of the present invention is the method for producing acathode material for a lithium battery according to the first or secondaspect, in which the compound which releases phosphate ions in asolution is phosphoric acid, phosphorous pentoxide, or ammoniumdihydrogenphosphate.

According to the method for producing a cathode material for a lithiumbattery, there can be obtained the effect, in addition to the effect ofthe first or second aspect, that impurities can be removed by thecalcination process since no nonvolatile element such as sodium iscontained in the ingredients, and ferric phosphate almost free ofimpurities can be synthesized from a stoichiometric mixture of theingredients. Thus, the ferric phosphate produced by the method can besuitably used as a cathode material for a lithium battery. Also, theingredients, which are primary materials or materials of the kind ofphosphoric acid and iron, are relatively inexpensive, easily availablein high purity form and easy to handle, and thus suitable forlarge-scale production.

The fourth aspect of the present invention is a method for producing acathode material for a lithium battery, including the steps of adding aconductive carbon to the cathode material produced by a method accordingto any one of the first to third aspects, and pulverizing and mixing themixture. According to the fourth aspect, since the ferric phosphateparticles as a cathode material are coated and combined with carbon, thelithium battery using the cathode material can be provided with animproved discharge capacity and higher performance.

The fifth aspect of the present invention is a lithium battery using acathode material produced by the method according to any one of thefirst to fourth aspects as a constituent component. The lithium batteryusing the cathode material produced by the method of the presentinvention exhibits high performance since the cathode material hasexcellent electrochemical properties.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the result of X-ray diffraction analysis of acathode material obtained in Example 1;

FIG. 2 is a graph showing the charge/discharge characteristics of asecondary battery obtained in Example 1;

FIG. 3 is a graph showing the charge/discharge characteristics of thesecondary battery obtained in Example 1;

FIG. 4 is a graph showing the charge/discharge characteristics of thesecondary battery obtained in Example 1;

FIG. 5 is a graph showing the result of X-ray diffraction analysis of acathode material obtained in Example 2;

FIG. 6 is a graph showing the result of X-ray diffraction analysis of acathode material obtained in Example 3;

FIG. 7 is a graph showing the result of X-ray diffraction analysis ofcathode materials calcined at different temperatures in Example 4;

FIG. 8 is a graph showing the charge/discharge characteristics ofsecondary batteries using the cathode materials calcined at differenttemperature in Example 4;

FIG. 9 is a graph showing the charge/discharge characteristics of asecondary battery obtained in Example 5; and

FIG. 10 is a graph showing the result of X-ray diffraction analysis of acathode material obtained in Example 6.

BEST MODE FOR CARRYING OUT THE INVENTION

The method for producing a cathode material for a lithium battery of thepresent invention is practiced by reacting a compound which releasesphosphate ions in a solution with metal iron in an aqueous solutionwhile grinding the mixture and calcining the reaction product.

<Cathode Material>

The cathode material obtained by the method of the present invention isferric phosphate represented by the general formula FePO₄. Ferricphosphate, which can be synthesized by reacting ingredients andcalcining the reaction product in the presence of air (in an oxidizingatmosphere), has a trigonal crystal structure with point group P₃₂₁ andcan be used as a cathode material for a lithium battery which can berepeatedly charged and discharged by insertion and extraction ofnegative metal ions such as lithium ions.

The ingredients of ferric phosphate as the cathode material of thepresent invention are a compound which releases phosphate ions in asolution and metal iron. The amounts of the ingredients are preferablyadjusted according to the stoichiometric ratio so that the mole ratio ofP and Fe can be 1:1.

Examples of the compound which releases phosphate ions in a solutioninclude but are not specifically limited to phosphoric acid (H₃PO₄),phosphorous pentoxide (P₂O₅), ammonium dihydrogenphosphate (NH₄H₂PO₄),and diammonium hydrogenphosphate [(NH₄)₂HPO₄]. Among these, phosphoricacid, phosphorous pentoxide, ammonium dihydrogenphosphate are preferredsince the iron is preferably kept under relatively strong acidicconditions during the process of dissolving.

When phosphoric acid is used as the ingredient, since phosphoric add isusually available in the form of aqueous solution, it is preferred tomeasure the phosphoric acid concentration (purity) precisely bytitration or the like before use. The metal iron is preferably in theform of fine particles (with a diameter of 200 μm or smaller, preferably150 μm or smaller, more preferably 100 μm or smaller) so that thereaction can be accelerated.

In the present invention, ferric phosphate as a cathode material for alithium battery can be easily obtained from a primary material such asmetal iron or the like as described above. Also, since the ingredientscontain no nonvolatile element such as sodium, impurities can becompletely removed only by calcination without cumbersome processes suchas filtering, and ferric phosphate almost free of impurities can besynthesized.

The reaction of the “compound which releases phosphate ions in asolution” with metal iron can be carried out by, for example, addingmetal iron to an aqueous solution of the “compound which releasesphosphate ions in a solution” that water is added as appropriate. In thereaction, it is necessary to fully dissolve the metal iron. In order todissolve the metal iron, grinding and/or heating refluxing or the like),for example, may be performed

That is, the process of reacting the “compound which releases phosphateions in a solution” with metal iron is preferably carried out by fullymixing and grinding the mixture of ingredients in an automatic grindingmachine, ball mill or beads mill and/or heating the mixture of theingredients by means of reflux or the like.

By grinding the mixture of ingredients, a shear force is applied to themetal iron and the surfaces thereof are renewed, accelerating thereaction. Hydrogen generated during the grinding process is preferablyremoved as appropriate. After the completion of the reaction, when thereaction product is dried, fine particles of ferric phosphate hydrate(with a diameter of about 1 μm or smaller) can be obtained. When thefine particles of ferric phosphate hydrate are calcined, fine particlesof a ferric phosphate cathode material with high activity can beobtained. To complete the reaction more fully, the reaction product maybe irradiated with ultrasonic waves. During the grinding process, whenthe generation of hydrogen in the initial stage decreases and thereaction is decelerated, the reaction is preferably carried out in thepresence of air or in an oxidizing atmosphere containing oxygen to expelhydrogen.

Also, when the reactants are heated, the dissolving reaction of themetal iron is accelerated and the yield of the cathode material can beimproved. Heating by refluxing or the like is preferably carried out inair to promote oxidation of the iron. Refluxing is considered to besuitable for large-scale production since there is no need for amechanical pulverizing process, which is relatively difficult to performin a large scale.

When the reaction is carried out in the presence of a reactionaccelerator, e.g. a volatile oxidizing agent such as hydrogen peroxide,oxygen, halogen such as bromine or chlorine, oxidized halogen such asbleaching powder or hypochlorous acid, or a volatile acid such as oxalicacid or hydrochloric acid, the reaction of the compound which releasesphosphate ions in a solution with metal iron may be accelerated andcompleted in a short period of time. However, when oxygen or oxidizingagent is added, since there is a danger of ignition, it is preferred totake precautions against explosion and control the vapor phasecomposition so that the concentration of it can be kept lower than theexplosion limit.

The addition of an oxidizing agent has the effect of convertinggenerated bivalent iron ions to tervalent iron ions as well as allowsthe metal iron to react with the compound which releases phosphate ionsin a solution and to dissolve (usually, as bivalent iron ions), so thatno bivalent iron can remain in the ferric phosphate cathode materialproduced by the calcination process after that. An acidic reactionaccelerator such as hydrochloric acid or oxalic acid has the effect ofaccelerating the hydrogen generating dissolving reaction of iron.Addition of a volatile reaction accelerator which is both oxidative andacidic such as nitric acid is also effective. Since the oxidizing agentand reaction accelerator are removed by the calcination process, thereis no possibility that they can remain in the cathode material.

By calcination of the reaction product of the compound which releasesphosphate ions in a solution and metal iron, ferric phosphate as thecathode material is produced. The calcination is carried out in acalcination process at temperatures ranging from 100 to 900° C. asgenerally employed under calcination conditions of suitable temperaturerange and treatment time. The calcination is preferably carried out inan oxidizing atmosphere containing oxygen such as air to promoteoxidation of iron.

The calcination carried out by, for example, a one-stage calcinationprocess including the steps of raising the temperature from roomtemperature to the calcination completion temperature (for example,about 100 to 900° C., preferably about 500 to 700° C. from the viewpointof removing water in the cathode material, more preferably about 650°C.) and keeping the temperature.

When the calcination is carried out at a low temperature of about 100 to500° C. (this process will be hereinafter referred to as“low-temperature calcination”), most of the ferric phosphate is in anamorphous state. When the calcination is carried out at a hightemperature of about 600 to 900° C. (this process will be hereinafterreferred to as “high-temperature calcination”), most of the ferricphosphate has a crystal structure with point group P₃₂₁. When thecalcination is carried out at a temperature in the range of 500 to 600°C. (this process will be hereinafter referred to as“intermediate-temperature calcination”), in which transformation fromthe amorphous phase to the crystal structure with point group P₃₂₁occurs, the proportion of amorphous phase decreases and ferric phosphatehaving a crystal structure with point group P₃₂₁ gradually increases asthe calcination temperature is higher, and the crystal structure withpoint group P₃₂₁ becomes dominant when the calcination temperature ishigher than 600° C. as described above.

When the electrochemical characteristics of the amorphous ferricphosphate obtained by low-temperature calcination, ferric phosphatehaving the crystal structure with point group P₃₂₁ obtained byhigh-temperature calcinations, and ferric phosphate in which amorphousphase and the crystal structure with point group P₃₂₁ coexist obtainedby intermediate-temperature calcination are compared, all of themexhibit relatively good discharge characteristics as described inExample 4 described later. The ferric phosphate obtained by low- orintermediate-temperature calcination shows a discharge curve which issimilar to that of the ferric phosphate obtained by high-temperaturecalcination although amorphous phase exists in them. This suggests thatthe local fine structure of the ferric phosphates is similar to thecrystal structure with point group P₃₂₁.

Thus, in the present invention, any of the low-, intermediate-, andhigh-temperature calcination can be selected, or the calcinationtemperature can be set in a low-intermediate temperature range (100 to600° C., for example) or an intermediate-high temperature range (500 to900° C., for example) depending on the ferric phosphate to be obtained.When the ferric phosphate is used in a non-aqueous electrolyte batterysuch as a lithium battery, since the cathode material preferably doesnot contain residual water, high-temperature calcination is preferredfrom the viewpoint of removing water completely.

The calcination is not limited to the one-stage calcination. Thecalcination may be carried out in two stages, that is, a calcinationstep in a lower temperature range (generally, in a range of roomtemperature to 300 through 400° C.; which may be hereinafter referred toas “preliminary calcination”) and a calcination step in a highertemperature range (generally, in a range of room temperature to thecalcination completion temperature (about 500 to 800° C., preferablyabout 500 to 700° C., more preferably about 650° C.); which may behereinafter referred to as “final calcination”). In such a case, it ispreferred that the reaction product of a compound which releasesphosphate ions in a solution and metal iron is preliminarily calcined inthe preliminary calcination step to obtain a calcination precursor andthe calcination precursor is kept in the above temperature range forabout 5 to 24 hours in the final calcination step. The reaction productmay be dried and/or pulverized as needed prior to the preliminarycalcination, and the calcination precursor may be pulverized and/orground prior to the final calcination. The one-stage calcination and theprocess including the preliminary and final calcination may be bothreferred simply to as “calcination.”

As has been described above, according to the method of the presentinvention, there is no need for a cumbersome filtering process in thesynthesis of the calcination precursor (ferric phosphate hydrate). Also,there is no possibility of impurities remaining after the calcination,and a cathode material almost free of impurities can be synthesizedreliably. In addition, the ingredients, which are primary materials ormaterials of the kind, are easy to handle and inexpensive and thussuitable for large-scale production.

When conductive carbon is added to the cathode material obtained asdescribed above and the mixture is mixed and ground for 12 to 36 hoursin, for example, a ball mill, a carbon composite cathode materialincluding cathode material particles coated with carbon can be obtained.One example of the conductive carbon is carbon black such as acetyleneblack.

When combined with carbon, the FePO₄ cathode material has significantlyimproved in discharge capacity as compared with the FePO₄ cathodematerial not combined with carbon as shown in Example 5 described later.That is, the combination with carbon improves the surface conductivityof the FePO₄ cathode material as a cathode active material andsignificantly enhances the utilization ratio of the positive activematerial. Thus, the combination with carbon is effective to improve theperformance of a lithium battery using FePO₄ as a cathode activematerial when carried out appropriately.

<Lithium Battery>

Examples of the lithium battery using the cathode material according tothe present invention obtained as described above include secondarybatteries such as metal lithium battery, lithium ion battery and lithiumpolymer battery. The lithium battery of the present invention can beused as a primary battery which is only discharged.

The basic structure of a lithium battery will be described taking ametal lithium battery using metal lithium as the anode material as anexample. A lithium metal battery is a secondary battery characterized inthat lithium ions move back and forth between the cathode and anodeduring charge and discharge by dissolution into the electrolyte anddeposition on the anode of metal lithium.

As the anode material, a compound containing lithium in the initialstate and having a central element in a reduced form such as lithiumcontaining alloys such as lithium-aluminum alloys, lithium-titaniumcomposite oxides (e.g., Li[Li_(4/3)Ti_(5/3)O₄]), and lithium-transitmetal composite nitride (e.g., Li₇MnN₄, Li₃FeN₂, etc.) as well as metallithium as used in metal lithium batteries can be used.

As the electrolyte, a liquid electrolytes prepared by dissolving anelectrolyte substance such as LiPF₆, LiBF₄, LiClO₄, LiCF₃SO₃,LiN(CF₃SO₂)₂, or LiC(CF₃SO₂)₃ in a mixed solvent of a cyclic organicsolvent such as ethylene carbonate, propylene carbonate, butylenecarbonate or γ-butyrolactone and a chain organic solvent such asdimethyl carbonate, ethyl methyl carbonate, diethyl carbonate ordimethoxyethane; a gel polymer electrolyte in which a liquid electrolyteas above and a polymeric gel substance such as polyethylene oxide,polypropylene oxide, polyacrylonitrile or polyvinylidene fluoridecoexist; or a crosslinked polymer electrolyte prepared by chemicalcrosslinking of a gel polymer electrolyte as above can be used. When theanode material is metal lithium, the use of a gel polymer electrolyte orcrosslinked polymer electrolyte which can suppress the growth ofdendrite which is deposited during charge is preferred. When a liquidelectrolyte is used, the cathode and the anode are insulated from eachother by interposing therebetween a separator made of a polyolefin suchas polyethylene or polypropylene to prevent short-circuiting betweenthem.

The cathode and anode are respectively prepared by adding a conductivityimparting agent, such as carbon black, in such an amount that theeffects of the cathode and anode are not impaired and a binder such as afluorine-type polymer, e.g., polytetrafluoroethylene or polyvinylidenefluoride; polyimide or polyolefin to the cathode or anode material,mixing and kneading the mixture with a polar organic liquid as needed,and forming the kneaded mixture into a sheet. Then, current collectionis conducted using a metal foil or metal screen to construct a battery.When metal lithium is used for the anode, transitions between Li(0) andLi⁺ take place upon charging and discharging, and a battery is therebyformed.

The secondary battery of the present invention produced by the abovemethod can exhibits high performance since the cathode material hasexcellent electrochemical properties. Especially, when the cathodematerial is used in a metal lithium battery using metal lithium for theanode, the battery exhibits good battery performance.

Although the following Examples will further described the presentinvention in more detail, the present invention shall not be limited bythese Examples.

EXAMPLE 1

(1) Preparation of Cathode Material

A cathode material (FePO₄) was synthesized by the following procedure.

An ingredient mixture of 6 g of iron powder (product of Wako PureChemical Industries, Ltd.; under 150 μm, purity: 85% or higher), 12.385g of phosphoric acid (product of Wako Pure Chemical Industries, Ltd.;85% aqueous solution) and 50 cc of water were mixed and ground in a ballmill (at a rotational speed of 200 rpm) for one day, and the reactionmixture was dried at 95° C. for one day to obtain a calcinationprecursor. After pulverization, the calcination precursor was placed inan alumina crucible and calcined at temperatures between 550 to 775° C.under the presence of air for 8 hours. A cathode material obtained bycalcination at 650° C. was identified as single-phase ferric phosphate(FePO₄) having a trigonal crystal structure with point group P₃₂₁ basedon the result of X-ray diffraction analysis shown in FIG. 1.

Diffraction Peaks Ascribable to Impurities Were Not Observed.

(2) Fabrication of Secondary Battery and its Charge/DischargeCharacteristics

The cathode material, acetylene black as a conductivity imparting agent[“Denka Black” (registered trademark); product of Denki Kagaku KogyoK.K, 50% pressed product], and PTFE (polytetrafluoroethylene) as abinder were prepared at a weight ratio of 70:25:5. The cathode materialand acetylene black were mixed and ground in a ball mill (at arotational speed of 200 rpm) for one day, and then mixed and kneadedwith the PTFE. The resulting mixture was formed into a sheet with athickness of 0.7 mm, and the sheet was punched out into disks with adiameter of 1.0 cm (area of 0.7854 cm²) to form a pellet as a cathode.

A metal titanium screen and a metal nickel screen were joined as cathodeand anode current collectors, respectively, to a coin-type battery casemade of stainless steel (CR2032) by spot welding. The cathode and ametal lithium anode were assembled in the battery case with a porouspolyethylene separator (Celgard 3501, a product of Celgard K.K.)therebetween. The battery case was filled with a suitable amount of 1 Msolution of LiPF₆ in a 1:1 mixed solvent of dimethyl carbonate andethylene carbonate as an electrolyte solution, and then sealed tofabricate a coin-type lithium secondary battery. All the assemblingprocess was performed in a dried argon-purged glove box.

The secondary battery in which the cathode material was incorporated wasrepeatedly charged and discharged at a current densities of 0.127 mA/cm²and 0.5 mA/cm² per apparent area of the cathode pellet in an operatingvoltage range of +2.0 V to +4.5 V. About one-hour open circuit state wasprovided at each switching between charging and discharging.

The charge/discharge characteristics in the first cycle at a currentdensity of 0.127 mA/cm² are shown in FIG. 2. The initial dischargecapacity was 132 mAh/g. The charge/discharge characteristic curve wasnot flat unlike that of a battery using an olivine-type lithium iron(II) phosphate (LiFePO₄) known as a cathode material for a secondarybattery.

The charge/discharge cycle characteristics at the same current densityare shown in FIG. 3. The discharge capacity decreased as the number ofcycle increased, and the lowest value was about 90 mAh/g.

The charge/discharge characteristics in the first to third cycles at acurrent density of 0.5 mA/cm² is shown in FIG. 4. The initial dischargecapacity was 112 mAh/g. The capacity gradually decreased during thethree cycles, and the lowest value was about 78 mAh/g.

When the cathode materials calcined at 550° C., 650° C. and 775° C.,respectively, were compared, the cathode material calcined at 650° C.showed the highest discharge capacity.

EXAMPLE 2

Preparation of Cathode Material

A cathode material (FePO₄) was synthesized by the following procedure.An ingredient mixture of 3 g of iron powder (product of Wako PureChemical Industries, Ltd.; under 150 μm, purity: 85% or higher), 6.1794g of ammonium dihydrogenphosphate (product of Wako Pure ChemicalIndustries, Ltd.) and 50 cc of water were mixed and ground in a ballmill (at a rotational speed of 200 rpm) for one day, and the reactionmixture was dried at 100° C. for one day to obtain a calcinationprecursor. After pulverization, the calcination precursor was placed inan alumina crucible and calcined at 650° C. under the presence of airfor one day. A cathode material obtained as described above wasidentified as single-phase ferric phosphate (FePO₄) having a trigonalcrystal structure with point group P₃₂₁ based on the result of X-raydiffraction analysis shown in FIG. 5. Diffraction peaks ascribable toimpurities were not observed.

EXAMPLE 3

Preparation of Cathode Material

A cathode material (FePO₄) was synthesized by the following procedure.An ingredient mixture of 11 g of iron powder (product of Wako PureChemical Industries, Ltd.; under 150 μm, purity: 85% or higher), 13.797g of phosphorous pentoxide (product of Wako Pure Chemical Industries,Ltd.) and 200 cc of water were mixed and ground in a ball mill (at arotational speed of 200 rpm) for one day, and the reaction mixture wasdried at 100° C. for one day to obtain a calcination precursor. Afterpulverization, the calcination precursor was placed in an aluminacrucible and calcined at 650° C. under the presence of air for one day.A cathode material obtained as described above was identified assingle-phase ferric phosphate (FePO₄) having a trigonal crystalstructure with point group P₃₂₁ based on the result of X-ray diffractionanalysis shown in FIG. 6. Diffraction peaks ascribable to impuritieswere not observed.

EXAMPLE 4

Preparation of Cathode Material

A cathode material (FePO₄) was synthesized by the following procedure.

200 ml of pure water was added to a stoichiometric mixture of ironpowder, 11.169 g (product of Wako Pure Chemical Industries, Ltd.; under150 μM, purity: 85% or higher) and phosphorous pentoxide, 14.483 g(product of Wako Pure Chemical Industries, Ltd.), and the resultingmixture was mixed and ground in a planetary ball mill at a rotationalspeed of 200 rpm for one day. The content was dried, and then dividedinto portions, which were calcined at 100° C., 200° C., 350° C., 500° C.and 650° C., respectively, in atmosphere for 12 hours. Each of theobtained cathode materials was pulverized in an agate mortar. Then, eachcathode material was formed into a cathode, and a coin-type lithiumsecondary battery was fabricated using a metal lithium anode in the samemanner as in Example 1.

The results of X-ray diffraction analysis of the synthesized cathodematerials are shown in FIG. 7. As is clear from FIG. 7, the samplescalcined at a temperature between 100° C. and lower than 500° C. had anamorphous structure with no diffraction peaks, and the sample calcinedat 500° C. was mostly amorphous but slightly crystallized into atrigonal crystal structure with point group P₃₂₁. On the other hand, thesample calcined at 650° C., the same calcination temperature as inExample 3, had a crystal structure with point group P₃₂₁.

The charge/discharge characteristics in the first cycle of the coin-typelithium secondary batteries using the cathode materials are shown inFIG. 8. The batteries were alternately charged and discharged between 2and 4V at a temperature of 25° C. and a current density per apparentarea of 0.2 mA/cm². Almost no difference in charge/discharge profile wasobserved between the amorphous and crystalline samples, and theirdischarge voltage profiles were apparently different from the flatprofile which lithium iron phosphate having an olivine-type crystalstructure (orthorhombic Pnma) showed, in which both oxidized and reducedforms coexist during charging and discharging reactions, and showedmonotonously decreasing curves which are seen in the case of homogeneousphase reaction.

In this example, when the charge/discharge characteristics were measuredon the samples synthesized from inexpensive starting materials of ironpowder and phosphorous pentoxide at synthesis temperatures between 100to 650° C., the samples calcined at temperatures of 350° C. or highershowed a maximum capacity of 115 mAh/g (see FIG. 8), which largelyexceeds the capacity of 40 mAh/g shown in the conventional report (citedbefore). Also, it should be noted that the cathode material obtained bycalcination at a very low temperature of 100° C. showed a dischargecapacity higher than 100 mAh/g.

Further, the results of X-ray diffraction analysis of the cathodematerials taken out of the coin-type lithium batteries after dischargingwere much the same as those immediately after production (the resultsare not shown), which indicates that the charge and discharge did notcause formation of a new phase. This indicates that the cathodematerials obtained in this example were all stable during charge anddischarge.

When TG (thermogravimetry) of the cathode materials was conducted,weight loss by thermal dehydration was hardly observed for the cathodematerials calcined at temperatures of 200° C. or higher. However,according to the result of Fourier infrared absorptionspectrophotometry, the absorption of the O—H—O deformation mode at 1600cm⁻¹ attributed to the existence of crystal water completely disappearedonly in the crystalline sample calcined at 650° C. To keep theperformance of a lithium battery stably over a long period of time, itis preferred that no water exist in the battery. Thus, from the point ofview of the long-term performance, calcination at 650° C. is consideredto be advantageous.

EXAMPLE 5

Acetylene black (product of Denki Kagaku Kogyo K.K, 50% pressed product)was added to the cathode material FePO₄ calcined at 650° C. and having acrystal structure with point group P₃₂₁ that was synthesized in Example4 in an amount of 25% by weight based on the total weight of themixture. The mixture was ground and mixed in a planetary ball mill at200 rpm for one day to obtain a cathode material including cathodematerial particles coated with acetylene black (which will behereinafter referred to as “carbon composite cathode material”). Then,the cathode material was formed into a cathode, and a coin-type lithiumsecondary battery was fabricated using a metal lithium anode in the samemanner as in Example 1.

A charge and discharge test was conducted on the coin-type secondarylithium battery. The result is shown in FIG. 9. The batteries werealternately charged and discharged between 2.6 and 4V at a temperatureof 25° C. and a current density per apparent area of 0.2 mA/cm². In FIG.9, the result of the test conducted on a cathode material calcined at650° C. but not combined with carbon (which was alternately charged anddischarged between 2.0 and 4V) is also shown.

FIG. 9 indicate that when combined with carbon, the FePO₄ cathodematerial had significantly improved discharge capacity as compared witha cathode material not combined with carbon and showed as high a valueas about 130 mAh/g. This is considered to be because the combinationwith carbon improves the surface conductivity of the FePO₄ cathodematerial as a cathode active material and significantly enhances theutilization ratio of the positive active material. This indicates thatthe combination with carbon is effective to improve the performance of alithium battery using FePO₄ as a cathode active material.

EXAMPLE 6

Preparation of Cathode Material

A cathode material (FePO₄) was synthesized by the following procedure.

200 ml of pure water was added to a stoichiometric mixture of ironpowder, 11.169 g (product of Wako Pure Chemical Industries, Ltd.; under150 μm, purity: 85% or higher) and phosphorous pentoxide, 14.483 g(product of Wako Pure Chemical Industries, Ltd.), and the resultingmixture was charged in a glass Erlenmeyer flask equipped with awater-cooled condenser and refluxed with a hot stirrer at 100° C. forthree days (this method in this Example will be hereinafter referred toas “reflux method”). The reaction mixture was taken out and dried, andthen calcined in atmosphere at 650° C. for 24 hours. The obtainedcathode materials was pulverized in an agate mortar. Then, the cathodematerial was formed into a cathode, and a coin-type lithium secondarybattery was fabricated using a metal lithium anode in the same manner asin Example 1.

The result of X-ray diffraction analysis of the synthesized cathodematerial is shown in FIG. 10. FIG. 10 indicates that it is possible toobtain FePO₄ having a crystal structure with point group P₃₂₁ by areflux method as in the case with the sample of Example 4 obtained bygrinding and reacting the ingredients in a planetary ball mill andcalcining the reaction mixture at 650° C.

A charge and discharge test was conducted on the coin-type lithiumsecondary battery using the cathode material under the same conditionsas in Example 4, a discharge voltage profile similar to that of thesample calcined at 650° C. shown in the top box in FIG. 8 and adischarge capacity of 115 mAh/g were obtained (illustration of themeasurement result is omitted).

This indicates that a calcination precursor equivalent to the oneobtained by grinding, dissolving and reacting the ingredients in aplanetary ball mill or the like can be synthesized by a reflux method,and that a high-performance cathode material can be obtained bycalcining the calcination precursor.

INDUSTRIAL APPLICABILITY

According to the method of the present invention, ferric phosphate(FePO₄) as a cathode material for a secondary battery can be producedreliably and easily. The cathode material produced by the method of thepresent invention is suitably used as a cathode material for a metallithium battery, for example.

1. A method for producing a cathode material for a lithium battery,comprising the steps of mixing a compound which releases phosphate ionsin a solution with metal iron to cause dissolution and reaction of themetal iron, and calcining the reaction mixture to synthesize ferricphosphate.
 2. A method for producing a cathode material for a lithiumbattery, comprising the steps of reacting a compound which releasesphosphate ions in a solution with metal iron while grinding the mixturein an aqueous solution, and calcining the reaction mixture to synthesizeferric phosphate.
 3. The method for producing a cathode material for alithium battery according to claim 1 or 2, wherein the compound whichreleases phosphate ions in a solution is phosphoric acid, phosphorouspentoxide, or ammonium dihydrogenphosphate.
 4. A method for producing acathode material for a lithium battery comprising the steps of adding aconductive carbon to the cathode material produced by the methodaccording to any one of claims 1 to 3, and pulverizing and mixing themixture.
 5. A lithium battery using a cathode material produced by themethod according to any one of claims 1 to 4 as a constituent component.